Yes, the door bottom is merely plastic. In the picture
with the cam and hinge plate removed, you can see that
beyond the plastic bottom is just insulating foam.
From what I can see, the plastic bottom only really
supports the door where the door metal sides meet the
plastic base at the edges. But, and here's the problem,
the plastic bottom is supported in its middle by the hinge
plate. This puts a lot of shear stress on the plastic
bottom.

On this refrigerator model, the door is designed to
hold four gallon-size milk jugs. I think this would
be roughly 32 pounds. Plus, there are two more
shelves. The door should be able to hold a good
bit of weight but that doesn't appear to be the case.

This ability to stowe milk jugs on the door was a selling
point when we purchased the refrigerator. Ironically,
we don't actually put milk jugs on our door but I'm
sure that other consumers do.

I'm still trying to think of ways this could have been
an installation problem. I don't think that is likely.
I could imagine an inept installer lifting up on the
door but I don't see any way why/how he would put
excessive downward force on the door. Plus, we never
noticed the door was lower than the other until very
recently. I have to believe this is a design
deficiency. Hopefully, my experience and quick fix
will help others as their refrigerators experience the
same problem.

The fabricated plate does make the door sit a wee bit
high but it is barely noticeable. I'm no machinist;
I'm sure others could fabricate a nicer plate than
mine. I made my plate by taking a piece of angle iron
(left over from my garage door installation), hammering
it flat and cutting it to fit with a hacksaw. I used
a drill to create holes where the screws go.